A known image forming apparatus, e.g., a laser printer, includes a fixing unit that is configured to fix a developer image transferred from a photosensitive member onto a recording sheet by heat. The fixing unit includes a heat roller that is subjected to heat from a heat source and receives power, and a pressure roller that is pressed against the heat roller.
To enhance the speed of image formation processing of the image forming apparatus, the fixing unit is used to enhance the speed of heat fixing. However, it is difficult to increase a surface temperature of the heat roller further in view of, for example, the melting point of a fluorine resin coated on the surface of the pressure roller. Thus, a nip width between the pressure roller and the heat roller is increased to extend a contact area between a recording sheet and the heat roller, to cope with high-speed heat fixing.
However, when the pressure roller is pressed against the heat roller, a part of the pressure roller that the heat roller contacts is deformed, e.g. dented, because the pressure roller is covered with an elastic layer. Such deformation of the pressure roller may increase a force of the pressure roller hindering sheet conveyance more than a force of the heat roller facilitating sheet conveyance, which may lead to a slippage between the heat roller and a recording sheet.
To solve this problem, the pressure roller may be forced to rotate while the heat roller is rotated. In this case, the outside diameter of the pressure roller changes because a surface (elastic layer) of the pressure roller expands with heat, and thus a peripheral speed of the pressure roller changes, which causes a difference in peripheral speed between the pressure roller and the heat roller, and results in a slippage between the pressure roller and the heat roller. Accordingly, a difference in sheet conveyance speed between both sides (a side facing the heat roller and a side facing the pressure roller) of a recording sheet occurs, which also causes a slippage between the heat roller and a sheet.
When the heat roller and the sheet slip, a trace of the heat roller may be left on the sheet, and a developer image on the sheet may be scraped against the heat roller, which may impair image quality.